The present invention relates to polycrystalline masses of self-bonded diamond or cubic boron nitride particles useful as tool components and, more particularly, to a metal-coated compact of polycrystalline diamond (PCD) or cubic boron nitride (CBN) that contains a second phase which can be bonded directly to a tool holder without the need for a cemented carbide support.
Diamond and the cubic form of boron nitride find use as abrasive materials in the form of (a) aggregated particles bonded by a resin or metal matrix, (b) cluster compacts, and (c) composite compacts. As bonded aggregates, particles of CBN or diamond abrasive are embedded in a grinding or cutting section of a tool such as a grinding wheel. The particles are typically coated with various metals and alloys of metals to enhance bond retention, oxidation resistance, graphitization resistance, and similar benefits. Representative art in the coating of single grains includes U.S. Pat. Nos. 2,367,404; 3,650,714; 3,957,461; 3,929,432; and 3,984,214.
A cluster compact is defined as a cluster of diamond or CBN crystals bound together in (a) a self-bonded relationship, (b) by means of a chemically bonded sintering aid or bonding medium, or (c) some combination of the two. U.S. Pat. Nos. 3,136,615 and 3,233,908 provide a detailed description of CBN cluster compacts which utilize a bonding medium and methods for making the same. U.S. Pat. No. 3,233,908 also describes self- bonded CBN compacts.
The diamond or cubic boron nitride of the cluster compact can be formed by converting graphite or HBN while simultaneously bonding the crystals formed. Therefore, cluster compacts can be made by (a) a one-step process in which a catalyst metal or alloy aids in the transition to an abrasive particle simultaneously with the formation of the compact, (b) a one-step process in which the abrasive particle is converted directly into a compact without the aid of a catalyst or bonding medium, or (c) a two-step process wherein the particles are formed first and subsequently bonded, with or without a catalyst, sintering aid, or bonding medium, to form a cluster compact.
Cluster compacts which contain residual metal from a catalyst, metal bonding medium, or sintering aid as a second phase are thermally sensitive and will experience thermal degradation at elevated temperatures. Cluster compacts which contain self-bonded particles, with substantially no secondary non-abrasive phase, are thermally stable. Their thermal stability enables such cluster compacts to be bonded directly to a tool holder by bonding methods such as brazing.
Cluster compacts which contain less than 3% non-diamond/non-CBN phase are described in U.S. Pat. Nos. 4,224,380 and 4,228,248. The compacts described in these patents are referred to as "porous compacts". Such compacts have pores dispersed there through in about 5-30% vol % of the compact. The porous compacts are made thermally stable by removal of the metallic phase by liquid zinc extraction, electrolytic depleting, or a similar process. These thermally stable porous composites have substantially no residual metal phase to catalyze back conversion or expand at a different rate from the surrounding abrasive. Because of the rough surfaces of these porous composite compacts, retention to a tool holder by a physical bond is suitable, and conventional brazing techniques can be used. These compacts have been coated so as to improve their oxidative stability when being bonded to a tool holder.
Cluster compacts which contain residual metal for a catalyst, metal bonding medium, or sintering aid as a second phase have been used effectively when part of a composite compact. A composite compact is defined as a cluster compact bonded to a substrate material such as cemented tungsten carbide. The bond to the substrate is formed under high pressure, high temperature conditions either during or subsequent to the formation of the cluster compact. Detailed disclosures of certain types of composite compacts and methods for making the same are found in U.S. Pat. Nos. Re. 32,380; 3,743,489; 3,767,371; and 3,918,219. The cemented substrate allows the composite compacts to be bonded to a tool holder by brazing or other conventional bonding methods. When part of a composite, a thermally sensitive cluster compact can, therefore, be bonded to a tool holder without damage.
The cemented tungsten carbide substrate of the composite is substantially larger in size than the abrasive bonded thereto. Therefore, a significant portion of the mass charged in the high pressure, high temperature apparatus is the substrate material, either before formation of the cluster compact or after. This volume of substrate reduces the amount of material which can be charged in the reactor to form the abrasive.
It is desirable to provide a method which allows cluster compacts with a metallic phase to be bonded to a tool holder without the need for a cemented tungsten carbide substrate.